Various packaging techniques have been used to build a load of unit products and subsequently wrap them for transportation, storage, containment and stabilization, protection and waterproofing. One system uses wrapping machines to stretch, dispense, and wrap packaging material around a load. The packaging material may be pre-stretched before it is applied to the load. Wrapping can be performed as an inline, automated packaging technique that dispenses and wraps packaging material in a stretch condition around a load on a pallet to cover and contain the load. Stretch wrapping, whether accomplished by a turntable, rotating arm, vertical rotating ring, or horizontal rotating ring, typically covers the four vertical sides of the load with a stretchable packaging material such as polyethylene packaging material. In each of these arrangements, relative rotation is provided between the load and the packaging material dispenser to wrap packaging material about the sides of the load.
Historically, wrappers have suffered from packaging material breaks and limitations on the amount of wrap force applied to the load (as determined in part by the amount of pre-stretch used) due to erratic speed changes required to wrap “non-square” loads, such as narrow, tall loads, short, wide loads, and short, narrow loads. The non-square shape of such loads often results in the supply of excess packaging material during the wrapping cycle, during time periods in which the demand rate for packaging material by the load is exceeded by the supply rate of the packaging material by the packaging material dispenser. This leads to loosely wrapped loads. In addition, when the demand rate for packaging material by the load is greater than the supply rate of the packaging material by the packaging material dispenser, breakage of the packaging material may occur.
When wrapping a typical rectangular load, the demand for packaging material varies, decreasing as the packaging material approaches contact with a corner of the load and increasing after contact with the corner of the load. When wrapping a tall, narrow load or a short load, the variation in the demand rate is even greater than in a typical rectangular load. In vertical rotating rings, high speed rotating arms, and turntable apparatuses, the variation is caused by a difference between the length and the width of the load. In a horizontal rotating ring apparatus, the variation is caused by a difference between the height of the load (distance above the conveyor) and the width of the load. Variations in demand may make it difficult to properly wrap the load. The problem with variations is exacerbated when wrapping a load having one or more dimensions that may differ from one or more corresponding dimensions of a preceding load. The problem may also be exacerbated when wrapping a load having one or more dimensions that vary at one or more locations of the load itself.
The amount of force, or pull, that the packaging material exhibits on the load determines how tightly and securely the load is wrapped. Conventionally, this wrap force is controlled by controlling the feed or supply rate of the packaging material dispensed by the packaging material dispenser. For example, the wrap force of many known commercially available stretch wrapping machines is controlled by attempting to alter the supply of packaging material such that a relatively constant packaging material wrap force is maintained. With powered pre-stretching devices, changes in the force or tension of the dispensed packaging material were monitored. This has been accomplished using feedback mechanisms typically linked to spring loaded dancer bars, electronic load cells, or torque control devices. The changing force or tension of the packaging material caused by rotating a rectangular shaped load is transmitted back through the packaging material to some type of sensing device which attempts to vary the speed of the motor driven dispenser to minimize the change. The passage of the corner causes the force or tension of the packaging material to increase. This increase is typically transmitted back to an electronic load cell, spring-loaded dancer interconnected with a sensing means, or to a torque control device. After the corner is passed, the force or tension of the packaging material reduces. This reduction is transmitted back to some device that in turn reduces the packaging material supply to attempt to maintain a relatively constant wrap force or tension.
With the ever faster wrapping rates demanded by the industry, the rotation speeds have increased significantly to a point where the concept of sensing changes in force and altering supply speed in response loses effectiveness. The delay of response has been observed to begin to move out of phase with rotation at approximately 20 RPM. The actual response time for the rotating mass of packaging material roll and rollers approximating 100 lbs must shift from accelerate to decelerate eight times per revolution that at 20 RPM is a shift more than every one half of a second.
Also significant is the need to minimize the acceleration and deceleration times for these faster cycles. Initial acceleration must pull against clamped packaging material, which typically cannot stand a high force especially the high force of rapid acceleration that cannot be maintained by the feedback mechanisms described above. Use of high speed wrapping has therefore been limited to relatively lower wrap forces and pre-stretch levels where the loss of control at high speeds does not produce undesirable packaging material breaks.
The present disclosure is directed to overcoming one or more of the above-noted problems, as well as other problems in the art.